1. Field of the Invention
The present invention relates to an apparatus for fabricating semiconductors that are used for fabricating semiconductor devices, in particular, to an apparatus for fabricating semiconductors, for example, an exposure apparatus provided with a prealignment mechanism for prealigning substrates, such as wafers or the like, which are to be exposed to exposure light, and to a method for fabricating devices using the same.
2. Description of the Related Art
In a conventional apparatus for fabricating a semiconductor or the like, prior to transferring a substrate to a substrate-treating stage, rough positioning (prealignment) is performed so that an alignment deviation of the substrate is brought within a given range. The rough positioning has been performed using a prealignment device having a mark detecting system which measures a mark on the substrate by detecting the mark before the substrate is transferred onto the substrate-treating stage.
In comparison with the final alignment accuracy that is required once the substrate is transferred to the stage, however, the alignment accuracy provided by the prealignment device is generally lower. Therefore, the substrate remains somewhat misaligned after the prealignment.
In order to compensate for the misalignment remaining after the prealignment, a more precise means for measuring the alignment marks, such as an image processor or the like, is generally provided close to the stage. After the prealigned substrate is placed on the stage, the alignment mark on the substrate is measured, and then the misalignment is compensated for with the substrate on the stage.
In the conventional technique described above, however, the misalignment can be measured only after the substrate is placed on the stage. As a result of the measurement, a misalignment may be determined to be a deviation of .delta. in a .theta. direction between an XY coordinate system of the stage and an XY coordinate system of the marks on the substrate. In this case, substrate treatment, such as an exposure treatment or the like, is performed after the misalignment is removed by either (i) holding the substrate again on the stage and correcting the relative position between the stage and the substrate by the deviation .delta. in the .theta. direction or (ii) rotating the stage by the deviation .delta. in the .theta. direction without holding the substrate again.
Once the substrate is held again, however, the duration of treating the substrate on the stage increases, resulting in a decrease in the throughput of the apparatus.
Also, the current position of the stage is typically measured by radiating a laser beam onto a mirror on the stage and measuring the reflected light. If the stage is rotated in the .theta. direction, the mirror for measuring the position is also rotated. The rotation of the mirror causes a deviation of the optical axis of the reflected laser light for measuring the position, which leads to a measurement error. This is referred to as Abbe error. Thereby, when the stage is rotated, the Abbe error must be compensated for in order to precisely measure the position. Although various approximations are used to compensate for the error, with an increase in the Abbe error, a difference between the approximate value and the theoretical value increases. That is, as the stage is rotated further in the .theta. direction, the positional measurement error of the stage caused by the Abbe error increases.
Also, if the stage is rotated in the .theta. direction, a deviation of the optical axis of the laser light reflected from the mirror occurs, as described above, and if the misalignment becomes too large, the reflected laser light cannot be measured, because the deviation of the optical axis becomes too great.
Also, in the conventional prealignment device, in spite of the fact that a deviation is calculated based on the position of the mark detecting system, there is no means for accurately determining the position of the mark detecting system itself, or for correcting the position of the mark detecting system. Thus, if the position of the mark detecting system changes with time, the detected deviation includes an error, resulting in an inaccuracy in the deviation from the reference position in the X and Y to directions and in the .theta. direction, with respect to the Z axis as a center.
The disadvantages referred to above will be described in detail with reference to FIG. 12 and FIG. 13. FIG. 12 is a schematic diagram wherein a mark detecting system lies in an ideal position. In the drawing, numeral 51 represents an expected position of a substrate held by a substrate carrier device (e.g., a carrying hand), numeral 52 represents an expected center of the substrate when at expected position 51, numeral 54 represents a center of the mark detecting system, numeral 55 represents a range to be measured by the mark detecting system, numeral 56 represents the position of a mark detected on the substrate, letter B represents a detected deviation of the substrate, numeral 53 represents the true position of the substrate as determined based on the detected deviation B, numeral 57 represents a position of a chuck, which is used as a device for holding the substrate before the deviation is corrected, numeral 58 represents a center of the chuck before the deviation is corrected, letter C represents a fixed operation length of the carrying hand, which is used for carrying the substrate from the mark detecting system to the chuck on the stage, numeral 60 represents a center of the chuck corrected in response to the detected deviation B, and numeral 61 represents an outline of the chuck, the position of which has been corrected in response to the detected deviation B.
In accordance with the conventional example, if the carrying hand holding the substrate is operated by an additional length corresponding to the detected deviation B when it is being operated to move by an amount corresponding to the fixed operation length C to place the substrate on the chuck, it is possible to accurately match up the center of the chuck with the center of the substrate.
However, as shown in FIG. 13, if the position of the mark detecting system itself changes to a position represented by numeral 62, the center of the mark detecting system deviates up to a position represented by numeral 54', and therefore, the positional deviation of the substrate will be changed to that shown by a differential B'. As a result, although the chuck is required to be moved up to the position represented by numeral 61 in order to accurately match up the center of the chuck with the center of the substrate, the chuck will actually be positioned at the position represented by numeral 63. That is, in spite of having performed a precise prealignment, the substrate is carried onto the device for holding the substrate with a wide deviation. Therefore, it may be necessary to perform the prealignment again, on the device for holding the substrate. Thus, the desired improvement of the throughput may not be achieved.